1. Field of the Invention
The present invention also relates to a display device for displaying images by inputting digital video signals. The present invention further relates to an electronic device using such a display device.
2. Description of the Related Art
In recent years, a so-called self-luminous display device in which a pixel is formed using a display element such as a light emitting diode (LED) has attracted attention. As a display element used for such a self-luminous display device, an organic light emitting diode (also referred to as an OLED, an organic EL element, an electroluminescent (EL) element, or the like) has attracted attention, and has been used for an EL display and the like. Since a display element such as an OLED is of a self-luminous type, it has advantages such as higher pixel visibility, no backlight required, and higher response speed compared to a liquid crystal display. Note that the luminance of the display element is controlled by the value of current flowing therethrough.
As a method for driving such a display device to display a grayscale, there are an analog grayscale method and a digital grayscale method. The analog grayscale method includes a method to control the light emission intensity of a display element in an analog manner and a method to control the light emission time of a display element in an analog manner. As the analog grayscale method, the method to control the light emission intensity of a display element in an analog manner is often used.
However, the method to control the light emission intensity in an analog manner is easily affected by variations in characteristics of a thin film transistor (hereinafter also referred to as a TFT) of each pixel, which causes variations also in luminance of each pixel. On the other hand, in the digital grayscale method, a display element is turned on/off by control in a digital manner to express a grayscale. In the case of the digital grayscale method, the uniformity of luminance of each pixel is excellent. However, there are only two states, that is, a light emitting state and a non-light emitting state, so that only two grayscale levels can be expressed. As another technique, there are an area grayscale method in which light emission area of a pixel is weighted and selected to perform grayscale display and a time grayscale method in which light emission time is weighted and selected to perform grayscale display. In the case of the digital grayscale method, the time grayscale method, which is also suitable to obtain higher definition, is often used (Reference 1: Japanese Published Patent Application No. 2002-6808)
Higher definition can be achieved by using the time grayscale method in the digital grayscale method. However, as definition proceeds, the number of pixels is increased. Therefore, the number of pixels to which a signal is written is also increased.
In addition, the number of subframes needs to be increased to perform high level grayscale display. Therefore, the number of signal writings into a pixel is increased.